The present system may be described as a pulverized coal boiler with an open cycle air turbine.
An air turbine may be defined as one in which clean air reaches the turbine as distinguished from a gas turbine in which combustion gases are introduced.
Studies have been made of fuel options for the near-term in the United States and it has been concluded that coal is the preferred energy source, particularly for electricity generation, see "Power" magazine, April 1982, pp. 128-132 and 350-354. That is because it is the most abundant energy resource, can be mined successfully and economically, thus is plentiful and low cost. An object of this invention is to use natural coal thereby avoiding expensive conversions of coal to synthetic liquid or gaseous fuels.
Conventionally, coal-fired steam boilers have been used with a steam turbine to generate electricity in power plants. However, the energy added for power generation is not all available for power because the heat of condensation of steam and the sensible heat of the condensed water are not useable for that purpose and constitute waste heat lost to the atmosphere except to the extent utilized for process steam, see "Cogeneration of Steam and Electric Power" edited by Robert Noyes, published by Noyes Data Corporation, Park Ridge, N.J., U.S.A., 1978, pp. 13-15. Consequently, the use of gas turbines--which do not have this drawback since steam is not being condensed--has come under consideration.
However, gas turbines themselves have certain limitations. In their operation, air is compressed, fuel is burned in the compressed air, and the combustion product gases are passed through the turbine. For that reason a premium quality fuel, which is either natural gas or a light petroleum distillate, must be used. If a fuel that is not clean-burning were used, the products of combustion, if passed through the turbine, would rapidly foul or damage the turbine passages and blades. Because of the fouling problem, the use of coal--which can evolve corrosive products and particulate matter--in open-circuit gas turbines has not reached a successful stage of development, although oil and gas find ready use, see "Analysis of Engineering Cycles", R. W. Haywood, Third Edition, 1980, pp. 5-8. Since the reserves of these fossil fuels are limited, practical commercial means of using coal in lieu thereof is highly desirable.
It has been suggested that since one of the major barriers to the widespread applications of gas turbines is that they require premium fuels, they are unlikely to gain wide market acceptance until they are able to burn gaseous or liquid fuels derived from coal (presumably clean fuels) see Noyes, ibid, p. 186. However, gas or liquid fuels derived from coal are not commercially available as yet and additionally this envisages a coal-derived fuel, so that work has to be done on the coal, thereby increasing the cost.
The present invention, going in a completely opposite direction, provides means whereby, in a practical and efficient manner, natural coal can be burned in a steam boiler as the energy source for an air turbine, and is therefore unobvious.
Some efforts have been made to employ combined cycles while using coal as energy source, viz., to heat the air for an air turbine by indirect heat exchange in order to solve the problem of corrosion of the turbine blades caused by direct contact with combustion product gases but these designs have not been attractive enough to receive market acceptance. In U.S. Pat. No. 3,127,744 to Nettel, the means for effecting such heat exchange is a pebble heater. Flue gas from a coal-fired steam boiler is further heated in a secondary furnace to raise its temperature. The flue gas is then passed into contact with pebbles, raising their temperature. Compressed air is then passed into contact with the hot pebbles and is then passed to the inlet of an air turbine. Pebble heaters are seldom used because of their cumbersome mechanical features which can lead to breakdowns, viz., the pebbles flow downwardly by gravity and at the bottom are hoisted up to the top by elevator means; and operation is cyclic in that half of the pebbles are being heated while the other half is releasing heat so that at any given time only half of the inventory is in use. Furthermore, the pebbles, at elevated temperatures, may produce corrosive products which can get into the air contacted by them and thus cause fouling of the turbine. It may be noted that Nettel uses an air heater which is outside the steam boiler.
U.K. Patent Application No. 2,027,739 discloses a process heater for hydrocarbon conversion. It is described as applied to the thermal cracking of hydrocarbons to produce ethylene, hence is a steam cracking furnace. The process heater is employed in conjunction with a gas turbine and the work obtained from the expansion zone is used for compression or refrigeration purposes in the plant. Following compression, the air is heated successively in a gas turbine recuperator, a furnace/turbine heat exchanger which transfers heat from flue gases in the bottom portion of the furnace stack to the air, and one or more conventional combustors. In such a combustor a premium fuel is burned in the air since the combustion products are passed into the turbine. The stack flue gas performs other duties essential to the hydrocarbon conversion process including preheating of the hydrocarbon feed, e.g., naphtha, and superheating of dilution steam for the steam cracking reaction. Under these circumstances not enough heat is available in the stack--because of other process heat requirements--to furnish enough heat to the compressed air to raise it to the high temperature levels achieved in the present invention. It should be noted that cycle efficiency increases with increase in temperature of the air at the inlet to the turbine, as shown in Table V in the following description. Consequently supplementary heating in a combustor is necessary for optimization of the process of this patent. The design therefore does not avoid the requirement to use a premium fuel for raising the temperature of the air. It will be noted that the patentee designates the turbine as a gas turbine. It will also be noted that coal is not used anywhere in the process since the pyrolysis furnace runs on fuel oil. Consequently there is no gain because all stages of heating the compressed air utilize a petroleum fraction as the fuel. In brief, U.K. No. 2,027,739 discloses an oil-fired process heater in conjunction with a gas turbine as distinguished from the present invention in which a coal-fired steam boiler is coupled with an air turbine.
An early patent, U.K. No. 563,847 accepted on Aug. 31, 1944, suggests that a power plant can comprise a steam turbine and an air turbine where the compressed air is heated in the coal-fired steam boiler. However, it does not disclose how the heat duties should be arranged to provide for the requirements of the air heater, since the radiant section continues its normal function of generating steam with the convection section doing all the superheating of the steam. Therefore, there is no teaching of how such a power plant could be designed or operated.
In U.K. Patent Application No. 2,066,284A a process of coal liquefaction is disclosed in which a coal slurry/hydrogen mixture is preheated to a moderate temperature in the range of about 750.degree. to 900.degree. by passing it through the convection section of a boiler. The combustion zone of the boiler burns a bottoms product of the coal liquefaction process itself. The unit furnishes process steam and surplus steam may be used to generate electricity. The application does not in particular describe the structure of the boiler. There is no disclosure of an air turbine. No suggestion is given of any means for operating a turbine which expands a gas so as to avoid the use of premium fuel and operate as an air turbine.